如何让复选框形状在CheckboxListTile中变圆？

ThemeData customTheme = ThemeData(
  checkboxTheme: CheckboxThemeData(
    shape: RoundedRectangleBorder(
      borderRadius: BorderRadius.circular(20.0),
    ),
  ),
);

Theme(
  data: customTheme,
  child: CheckboxListTile(
    title: Text('选项'),
    value: _isChecked,
    onChanged: (bool value) {
      setState(() {
        _isChecked = value;
      });
    },
  ),
)